1. Field of the Invention
The present invention relates to a method for producing a thin IC chip that is comprised of a thinned wafer and a reinforcing plate, particularly to a method for producing a non-contact type IC card.
2. Description of the Related Art
Silicon wafers of thin film types, which have been introduced into non-contact type IC cards, have been applied to various fields, such as ticket gates of railroads and bankcards. It is expected that thin-film silicon chips will be further applied to various fields, such as IC tags, in the future. A thin-film silicon chip is bonded to a reinforcing plate, which is made of a thin plate of stainless steel or the like, so that the chip is not easily damaged by external forces, because a thin-film silicon chip is extremely brittle in itself.
Various methods for producing IC chips have been proposed. According to a conventional art, first, grooves are formed in a wafer, and then the wafer is diced. In this method, first, grooves that are deeper than the thickness of the finished wafer are formed along chip portions that are formed on the wafer. Then, the wafer is attached to a base plate with an adhesive tape, wax, or the like. The wafer is ground from the back surface to a predetermined thickness and is separated into the chip portions. The separated chip portions are attached to reinforcing plates to produce IC chips. However, this method has the disadvantage that machining stress and strain are caused in the wafer when the back surface is ground, and the chip portions are apt to experience cracks due to external forces. Further, the characteristics of an IC chip, as an element, tends to be more affected as the wafer becomes thinner. Since the chip portions that are separated need to be attached to respective reinforcing plates one by one, time-consuming and low productivity work is required. Grinding and cleaning the back surface of the chip portion is also required for each separated piece.
The stress and strain, which is generated in the back surface of the wafer, can be removed by using physical means, such as milling, or by using wet etching, after the back surface is ground. Specifically, this method is carried out by the following steps. First, a wafer is attached to a base plate, and the wafer is ground. At this step, the wafer is not yet separated into chip portions. After the wafer is ground, the surface that has been ground is etched by wet means using hydrofluoric acid or the like, by milling in vacuum, or by RIE (Reactive Ion Etching) in order to remove the processing strain in the surface that has been ground. It is not preferable that etching is performed after the wafer is separated into chip portions, because the side surfaces of each piece may be damaged by etching.
The wafer, which is thinned and the surface of which is ground and etched, is attached to a base plate, a sheet, or the like, to be diced into chip portions. The separated chip portions are attached to a metal plate one by one. The metal plate is diced to form IC chips. Alternatively, the separated chip portions may be bonded to respective reinforcing plates one by one. However, these methods have the disadvantage that productivity is low.
In order to overcome this disadvantage, a thinned wafer and a reinforcing plate may be bonded together, and the combination thereof may be diced to form each IC chip. However, if the combination of the reinforcing plate and the wafer which are in contact with each other via an uncured adhesive is removed from an apparatus, and a plurality of the combinations are put into a high-temperature furnace together, then the wafer may easily be damaged by external forces, because the wafer that has been thinned and that is held on the reinforcing plate by the uncured adhesive is extremely brittle. Further, since it is difficult to apply the adhesive evenly in a wide area, it is difficult to bond the wafer and the reinforcing plate together while completely preventing air bubbles from entering the adhesive layer. Bubbles often cause chippings that originate from the air bubbles or cracks in a wafer during dicing, leading to worsened yields. Further, in general, metals used for the reinforcing plate, such as stainless steel, are diced with a dicing blade in a different manner as compared to a silicon wafer etc. Therefore, when the combination of the reinforcing plate and the wafer is diced simultaneously, the number of chippings in the wafer may increase, and metal powders may often clog the blade, resulting in serious damage to the surface of the diced wafer. Reduction in the dicing speed of the blade for reducing damage to the surface may significantly worsen productivity. If a reinforcing tape is attached to a wafer prior to dicing in order to dice the wafer in a reinforced state, then a step of peeling the reinforcing tape is necessary after dicing.